Some people say it is not advisable to do achamana facing west, so one should face north while doing evening Sandhyavandanam up to Navagruha Tarpanam and after that one should do facing west till Namaskaram and then finish Sandhyavandanam facing north. But some others say one can even do the entire Sandhayavandanam facing west. Which is true?
The book Nitya Karma Puja Prakash quotes the following verse from the Devi Bhagavatam, according to which, the evening Sandhya should be done facing the west.
In evening as long as the stars don't rise in the sky one should worship Sandhya facing the west. In morning one should face east and worship till one can see the risen Sun.
But, irrespective of which direction you are facing while performing Sandhya, Achamana must always be done only while facing east or north.
Manu Smriti 2.61. He who knows the sacred law and seeks purity shall always perform the rite of sipping with water neither hot nor frothy, with the (prescribed) tirtha, in a lonely place, and turning to the east or to the north
If Achamana is performed facing west/south then that's considered an offence and a Prayaschitta is required.
The book "Dharma Bindu" (on page 32) have this Prayaschitta for an Achamana which is performed facing west:
Na bahurjaanuraachamet naasanasthona chotthitah,
Bhuktvaasanasthopya achaame anya kaale kadachana/
Naanteerayaika deshasya krutvaachavottareeya kam,
Aacchanna dakshinaam sastu naachaametthu kadaahana/
Vinaa yagjnopaveetena tathaadhoutena vaasanaa,
Muktvaa shikhaam vaachantena kritasvaiva punah kriya/
Yaabhya pratyagmu khatvena kritama - achamanam yadi,
Praayas- chityai tadaa kuryaat snaanamaachamanam kramaat/
‘Achamana’ is required to be performed without being seated or streching hands outside the knees. Only after bhojana, one can seated on a seat but not otherwise. Achamana should not be done with dhoti or the main garment and the upper garment together, or covering the right shoulder. Without yagnopaveeta and open up the tuft while performing ‘achamana’ is banned. Achamana should not be performed west faced and if so done prayaschitta or atonement is required by taking bath and perform ‘Punara-acham
So, after performing the rite of Achamana facing east or north, one has to change his direction accordingly.